A synthetic rubber-based hot melt adhesive containing a thermoplastic block copolymer as a main component has widely been used as a hot melt adhesive which is used in disposable products such as a diaper and a napkin and is applied to a base material thereof, for example, a nonwoven fabric, a tissue, a polyethylene film and the like.
JP 2004-137297 A discloses that a synthetic rubber-based hot melt adhesive containing a styrene-butadiene-styrene block copolymer as a main component can be employed in disposable products. In the case of producing disposable products, a base material such as a film or a nonwoven fabric is coated with a hot melt adhesive. In order to enhance production efficiency of the disposable products, a coating speed may be sometimes increased, that is, coating may be sometimes performed at a high speed. In particular, it is necessary to increase a discharge amount of the hot melt adhesive so as to coat at a high speed in the case of performing spiral coating. Therefore, it is necessary to set a pressure of hot air to be blown to a high value. However, when the synthetic rubber-based hot melt adhesive of JP 2004-137297 is sprayed under a high hot air pressure, the hot melt adhesive may be sometimes scattered.
One of means for enhancing production efficiency of the disposable products includes a method in which an olefin-based hot melt adhesive typified by an ethylene-propylene copolymer is applied at a high speed.
WO 2001-094690 A discloses that a propylene polymer can be used as a raw material for a hot melt adhesive. The polyolefin-based hot melt adhesive of WO 2001-09490 A develops high adhesive strength in the case of bonding a nonwoven fabric and a nonwoven fabric together. However, the adhesive is insufficient for the disposable products because of insufficient adhesion of the adhesive to a polyethylene film.
The disposable products such as a diaper and a sanitary napkin often have a structure in which an absorber constituted by a pulp, an absorbent polymer and the like is wrapped in a tissue and the outside of the tissue is covered with a nonwoven fabric, a polyethylene film and the like. Therefore, it is required for the hot melt adhesive for disposable products to have a strong adhesive strength to the nonwoven fabric and the polyethylene film.
JP 2009-242533 A discloses a hot melt adhesive containing a hydrogenated styrene-isoprene block copolymer (SEPS) and a polyolefin. The hot melt adhesive of JP 2009-242533 A shows satisfactory adhesion to a nonwoven fabric, but shows poor adhesion to a polyethylene film because of rigid design containing no oil component in principle, and thus it is hard to use in disposable products. Furthermore, since the adhesive of JP 2009-242533 A contains an amorphous polyolefin including a high-molecular weight component, the adhesive cannot obtain a sufficient width in spiral coating and has high viscosity. Therefore, the adhesive of JP 2009-242533 A was not suited for spiral coating to a polyethylene film at low temperature. In order to cope with a complicated coating pattern, it is necessary to decrease the viscosity by increasing a coating temperature. However, when the coating temperature is increased, the film as the base material may be melted and the coated hot melt adhesive may shrink due to cooling, resulting in the formation of wrinkles in the film.
JP 2002-519474 A discloses a hot melt adhesive obtained by mixing an ethylene/α-olefin interpolymer with a block copolymer. Example 14 of JP 2002-519474 A discloses a hot melt adhesive for disposable products obtained by mixing an ethylene/α-olefin interpolymer with a hydrogenated styrene block copolymer. Since this hot melt adhesive is obtained by blending the hydrogenated styrene block copolymer, adhesion to a polyethylene film is improved. However, since low cohesive force of the ethylene/α-olefin interpolymer leads to low adhesion between nonwoven fabrics, it is difficult to mention the hot melt adhesive as being a sufficient adhesive for disposable products.